在Makefile中可以使用函数来处理变量，从而让我们的命令或是规则更为的灵活和具有智能。make所支持的函数也不算很多，不过已经足够我们的操作了。函数调用后，函数的返回值可以当做变量来使用。-Can use the function in the Makefile to deal with variables, so that commands or rules are more flexible and intelligent. make the support function is not too many, but enough for our operations. Function call, function return value can be used as a variable to use.